A fuel cell system can generate electric energy with high efficiency by a reaction between a hydrogen-rich fuel generated by reforming of a raw material and oxygen in air as an oxidizing agent. It is known that the activity of a catalyst used for the reforming of the raw material significantly deteriorates by sulfur poisoning. Therefore, a sulfur component needs to be removed from the raw material before the raw material is supplied to a reformer.
As a method of removing the sulfur compound in the raw material, there are an ordinary temperature desulfurization in which adsorptive desulfurization is performed by an adsorbent at ordinary temperatures and a hydrodesulfurization in which the sulfur compound is reacted with hydrogen on a hydrodesulfurization catalyst to generate hydrogen sulfide, and the hydrogen sulfide is removed by adsorption.
Since the ordinary temperature desulfurization does not require heating, a system configuration thereof is simple. However, since an adsorption capacity of a desulfurizing agent is not large, the desulfurizing agent needs to be replaced periodically.
In contrast, in the hydrodesulfurization, the adsorption capacity is large. Therefore, the system of the hydrodesulfurization can be configured such that the desulfurizing agent does not have to be replaced even in the case of a long time operation. However, before the raw material containing the sulfur compound is supplied to the hydrodesulfurization catalyst, the raw material needs to be mixed with the hydrogen, and the hydrodesulfurization catalyst needs to be heated to about 300° C.
Typically, a home fuel cell system uses a method of circulating a part of the hydrogen-containing gas generated by the reformer, as a source of the hydrogen that is mixed with the raw material. According to this method, before the hydrogen-containing gas is generated by the reformer, there is no hydrogen. Therefore, to avoid the sulfur poisoning of a reforming catalyst during this period of time, for example, an adsorbent for adsorptive desulfurization is provided.
To circulate a part of the hydrogen-containing gas generated by the reformer, a method of disposing a transfer device, such as a blower, on a recycle passage or a method of adjusting a pressure balance by providing, for example, an orifice at an upstream side of a merging portion, at which a raw material supply passage and a hydrogen-containing gas circulation passage merge with each other, such that the pressure of the hydrogen-containing gas becomes higher than the pressure of the merging portion.
In the home fuel cell system of about 1 kW, the operating temperature of a fuel cell is about 80° C. or lower. Therefore, there is a possibility that steam in the circulating hydrogen-containing gas condenses, and the condensed water is supplied to a booster configured to supply the raw material.
Here, proposed as a method of circulating the hydrogen-containing gas generated by the reformer is a method of: causing a part of the hydrogen-containing gas generated in the reformer to flow through a condenser to remove the steam from the hydrogen-containing gas; and then causing the hydrogen-containing gas to merge with the raw material supply passage provided upstream of the booster (see PTLs 1 and 2, for example).